1. Field of the Invention
The invention relates generally to a corrosion resistant coating, and in particular, to a coating for use on a component in a corrosive environment of a semiconductor wafer processing system.
2. Description of the Background Art
In a semiconductor wafer processing system, the interior of a processing chamber is often exposed to a variety of corrosive or reactive environments. These reactive environments may result from either corrosive stable gases, e.g., chlorine (Cl2), or other reactive species, including radicals or by-products generated from process reactions. In plasma process applications such as etching or chemical vapor deposition (CVD), reactive species are also generated through dissociation of other molecules, which by themselves, may or may not be corrosive or reactive. Corrosion resistant measures are needed to ensure process performance and durability of the process chamber or component parts. Nickel-plated components, for example, are often used in process chambers to prevent corrosion from Cl2. Fluorine-containing gases such as NF3 or CHF3, among others, give rise to atomic fluorine (F) which is highly reactive. Corrosion becomes even worse under high temperature environments, such as those encountered in certain CVD applications. For example, ceramic heaters made of aluminum nitride (AlN) are attacked by NF3, which is often used as a cleaning gas in some wafer processing systems. These heaters are typically rather expensive, and it is desirable to have protective coatings which can prevent corrosion of the heater surfaces.
Therefore, a need exists in the art to provide corrosion resistant coatings that can protect process components, such as ceramic heaters, from corrosion within wafer processing systems.
The present invention provides a protective coating upon a component part for use in a corrosive or reactive environment. The protective coating comprises magnesium fluoride that is sufficiently pure to avoid reactions between contaminants in the coating and reactive species in the surrounding environment. Furthermore, the coating is sufficiently dense to preclude reactive species from penetrating the coating and reaching the underlying component part. In one embodiment, the magnesium fluoride coating is at least 85% dense, e.g., about 85-90% dense, and at least 99% pure. Such a coating, for example, protects a ceramic heater surface against chemical attack by fluorine radicals in a high temperature corrosive environment.
In another preferred embodiment, the component part (e.g., aluminum nitride heater) is provided with a surface finish better than about 10 RA, or preferably, between 5-10 RA. The coating formed upon such a component part is more uniform, and thus provides more effective protection against corrosion.
The quality of the protective coating is also affected by the process condition of the deposition. Therefore, another aspect of the invention is a method of forming the coating at a temperature and pressure that are appropriate for the desired coating density and purity. In general, it is preferable that the coating be formed at a relatively high temperature and relatively low pressure. A high deposition temperature tends to yield a higher density coating, while a low pressure results in a coating with a higher purity. In one preferred embodiment, the coating is deposited at a temperature of at least about 250xc2x0 C., or preferably at least about 300xc2x0 C., and a chamber pressure of lower than about 1xc3x9710xe2x88x925 torr. Further improvement in the coating characteristics can be achieved by annealing the coating at a temperature greater than about 600xc2x0 C.